The present invention relates generally to sheet material and more specifically to sheet material having projections on its surfaces.
As referred to herein, sheet material of the kind specified refers to sheet material having on both of its faces a plurality of rows of projections, each projection having been formed by deforming the sheet material locally to leave a corresponding depression at the opposite face of the material. This deformation is effected by a forming tool and results in both plastic strain hardening and in an increase of the effective thickness thereof. Sheet material of the kind specified is stiffer than the plain sheet material from which it is formed and the mass of material required for a particular duty can be reduced by using sheet material of the kind specified in place of plain sheet material.
The magnitude and distribution of plastic strain exerted on the sheet material depends on a number of factors including, inter alia, the depth of penetration of the forming portions of the tool and the geometry of the forming portions.
An example of sheet material of the kind specified is disclosed in EP0674551, which is owned by the current applicant, wherein the sheet material is provided with the relative positions of the projections and depressions such that lines drawn on a surface of the material between adjacent rows of projections and depressions are non-linear. The projections are formed by forming tools having teeth with four flanks, wherein each flank faces a direction between the axial and circumferential directions of the rolls.
A further factor which affects the magnitude and distribution of plastic strain in such an arrangement is the layout or concentration of teeth in the forming tool.